Field of the Invention
This invention relates to marine seismology and more particularly relates to the use of an underwater vehicle for the touchdown monitoring of the deployment of a plurality of ocean bottom seismic nodes.
Description of the Related Art
Marine seismic data acquisition and processing generates a profile (image) of a geophysical structure under the seafloor. Reflection seismology is a method of geophysical exploration to determine the properties of the Earth's subsurface, which is especially helpful in determining an accurate location of oil and gas reservoirs or any targeted features. Marine reflection seismology is based on using a controlled source of energy (typically acoustic energy) that sends the energy through seawater and subsurface geologic formations. The transmitted acoustic energy propagates downwardly through the subsurface as acoustic waves, also referred to as seismic waves or signals. By measuring the time it takes for the reflections or refractions to come back to seismic receivers (also known as seismic data recorders or nodes), it is possible to evaluate the depth of features causing such reflections. These features may be associated with subterranean hydrocarbon deposits or other geological structures of interest.
In general, either ocean bottom cables (OBC) or ocean bottom nodes (OBN) are placed on the seabed. For OBC systems, a cable is placed on the seabed by a surface vessel and may include a large number of seismic sensors, typically connected every 25 or 50 meters into the cable. The cable provides support to the sensors, and acts as a transmission medium for power to the sensors and data received from the sensors. One such commercial system is offered by Sercel under the name SeaRay®. Regarding OBN systems, and as compared to seismic streamers and OBC systems, OBN systems have nodes that are discrete, autonomous units (no direct connection to other nodes or to the marine vessel) where data is stored and recorded during a seismic survey. One such OBN system is offered by the Applicant under the name Trilobit®. For OBN systems, seismic data recorders are placed directly on the ocean bottom by a variety of mechanisms, including by the use of one or more of Autonomous Underwater Vehicles (AUVs), Remotely Operated Vehicles (ROVs), by dropping or diving from a surface or subsurface vessel, or by attaching autonomous nodes to a cable that is deployed behind a marine vessel.
Autonomous ocean bottom nodes are independent seismometers, and in a typical application they are self-contained units comprising a housing, frame, skeleton, or shell that includes various internal components such as geophone and hydrophone sensors, a data recording unit, a reference clock for time synchronization, and a power source. The power sources are typically battery-powered, and in some instances the batteries are rechargeable. In operation, the nodes remain on the seafloor for an extended period of time. Once the data recorders are retrieved, the data is downloaded and batteries may be replaced or recharged in preparation of the next deployment. Various designs of ocean bottom autonomous nodes are well known in the art. Prior autonomous nodes include spherical shaped nodes, cylindrical shaped nodes, and disk shaped nodes. Other prior art systems include a deployment rope/cable with integral node casings or housings for receiving autonomous seismic nodes or data recorders. Some of these devices and related methods are described in more detail in the following patents, incorporated herein by reference: U.S. Pat. Nos. 6,024,344; 7,310,287; 7,675,821; 7,646,670; 7,883,292; 8,427,900; and 8,675,446.
When a plurality of autonomous nodes attached to a cable, or an ocean bottom cable, is deployed in the sea, it is desirable to know the position of the cable and the positions of the nodes during and after deployment. One common way to identify some portions of the deployed cable is to use acoustic positioning transponders that are selectively placed at various portions of the cable. The transponders may transmit an acoustic signal to a marine vessel that indicates the position of the transponders on the sea floor, which indications the position of selected portions of the cable. However, many problems exist with the use of transponders. Because transponders are expensive, they are used infrequently on segments of the deployment cable, often at intervals of 300 meters or more. In operation, cable is laid down not in perfect lines or patterns on the seabed, and thus the exact location of entire sections of cable (and the relevant seismic nodes) are effectively unknown between the transponders. Further, communications using transponders is very limited and does not provide the needed accuracy or real-time communications ability to guide the positioning of the deployed cable as it is being deployed. Still further, the use of transponders provides no clear evidence data that touchdown (i.e., the point of contact of a node to the seabed) has effectively occurred, particularly as to each node. Recording seismic signals from the seabed requires proper positioning of the node and/or sensor on the seabed and effective seismic coupling between the node and the seabed. Different orientations and improper configurations inhibit the coupling of the seismic sensor to the seabed, providing poor or inaccurate data. Still further, the deployment and retrieval of transponders requires additional equipment on the deployment vessel and additional time for the handling of such transponders.
A need exists for an improved method and system for the monitoring and/or guiding of cable deployed with nodes on the seabed, and in particular one that eliminates all or substantially all of the transponders typically used in such applications. A new system is needed that is more cost effective, allows better positioning and accuracy of deployed nodes, allows for the real-time (or near real-time) guidance of the deployment cable, and provides confirmation that effective touchdown has occurred.